The use of focused pulsed laser radiation for the purpose of generating incisions in the corneal tissue or in other tissue parts of the human eye has been the subject of intense research in human ophthalmology for some time. Instruments are also already on the market that provide a function of incision generation with laser radiation of such a type. Ordinarily in this connection, ultra-short-pulse laser radiation with pulse durations within e.g. the femtosecond range finds application. However, the invention is not restricted to this; to the extent that generation of an incision in corneal eye tissue is possible also with shorter or longer pulse durations, these are likewise to be encompassed by the invention; for example, pulse durations within the attosecond range or within the one-digit, two-digit or three-digit picosecond range.
A physical effect that is utilised in the course of the generation of an incision by means of pulsed laser radiation is the so-called laser-induced optical breakthrough, which results in a so-called photodisruption, the magnitude of which is limited roughly to the extent of the radiation focus at the waist point of the radiation. As a result of juxtaposing a plurality of such photodisruptions, diverse and comparatively complex incision figures can be generated in the eye tissue.
An exemplary application of the generation of an incision by means of pulsed laser radiation is so-called LASIK (laser in-situ keratomileusis). In this surgical procedure—which is generally to be classified as refractive surgery, that is to say, surgery aimed at the elimination or at least improvement of defective imaging properties of the eye—firstly the human cornea is cut open horizontally (from the point of view of the reclining patient), whereby a small cover (ordinarily called a flap in specialist circles) arises which can be folded aside. After the flap has been folded aside, in the stroma of the cornea that has been exposed in this way a so-called ablation is effected by means of laser radiation (for example, excimer radiation with a wavelength of 193 nm), i.e. stromal tissue is removed in accordance with a suitable ablation profile computed beforehand for the patient. After this the small cover is folded back, the healing process proceeding comparatively painlessly and quickly. After this intervention the cornea has different imaging properties, in which connection a very largely total elimination of the prior defective vision is achieved in the best case.
In the prior ‘classical’ procedure the cutting of the flap is effected with a mechanical microkeratome, in which connection, however, cutting the flap using laser technology has also recently been contemplated. The existing conceptions for this frequently provide for an applanation (levelling) of the anterior surface of the cornea by abutment against a planar abutment surface of a contact element that is transparent to the laser radiation, the flap then being generated by a bed incision situated at constant depth and by a lateral incision extending from the bed incision as far as the surface of the cornea. The levelling of the cornea permits the bed incision to be executed as a two-dimensional incision, for which solely a control of the location of the radiation focus in a plane perpendicular to the direction of propagation of the radiation (designated in conventional notation as the x-y plane) is required, without undertaking a control of the location of the radiation focus in the direction of propagation of the laser radiation (this direction is designated, according to conventional notation, as the z-direction). For the generation of the bed incision, the radiation focus is moved, for example, along a meandering scan path, i.e. a tortuous path that is composed of a plurality of rectilinear path portions situated next to one another in parallel which are connected to one another at their ends by means of redirecting-portions which are curved in arcuate manner or angular. For the generation of the lateral incision, the radiation focus is moved, for example, along a helical or spiral path ascending from the bed incision to the surface of the cornea, or along several superposed circular paths. Since at invariable pulse repetition rate the spacing of consecutive radiation pulses in the reversing-portions of the meandering scan path of the bed incision may decrease, in PCT/EP 2009/003730 a selective blanking is proposed of radiation pulses that are situated in regions of the meandering scan path that lie outside the lateral incision. By this means, thermal damage in the reversing-portions of the meandering scan path is intended to be avoided.
Another form of operation in which incisions are generated in the cornea by means of pulsed laser radiation is laser-assisted corneal lenticle extraction. In this case, in the stroma of the cornea a tissue volume—which, for example, has the shape of a small disc—is cut free which can then be extracted from the eye through an auxiliary incision. Depending on the indication (e.g. myopia, hyperopia), the lenticle to be removed may have varying shape. For the purpose of cutting the lenticle free, the procedure hitherto has frequently been such that firstly a lower incision bounding the underside of the lenticle (posterior side of the lenticle) and subsequently an upper incision bounding the upper side of the lenticle (anterior side of the lenticle) are generated in the cornea, both incisions frequently being three-dimensional and each requiring a z-control of the radiation focus. For both incisions the radiation focus is moved, for example along a meandering scan path, whereby at each point of the meandering scan path the z-position of the radiation focus is set to the position of the incision in question. During the scanning of the meandering scan path it may accordingly be necessary, time and time again, to adjust the radiation focus in the z-direction, in which connection under certain circumstances this may be necessary continuously from radiation pulse to radiation pulse.
A similar procedure can frequently also be noted in the case of the generation, using laser technology, of corneal keratoplasty incisions, i.e. incisions by which a piece of corneal tissue that is diseased or injured, and therefore to be transplanted, or a piece of corneal tissue of a donor eye serving as donor material is cut free. To be mentioned especially in this connection are endothelial and epithelial keratoplasty incisions. In the case of corneal keratoplasties, the requisite keratoplasty incisions may occasionally be considerably complex. This gives rise to comparatively frequent z-adjustments of the radiation focus if an attempt is being made to generate a three-dimensional incision with a single meandering scan path.
With a view to x-y adjustment of the radiation focus, sufficiently fast scanners are available which, for example, operate with galvanometrically controlled scanner mirrors. On the other hand, available z-scanners—that is to say, scanners that enable a focus displacement in the z-direction—are frequently slow in comparison with galvanometric mirror scanners. Depending on the complexity of the shape of the incision to be generated, i.e. depending on the extent of the z-focus displacements to be executed when sweeping the surface defining the incision, the requisite period of time for the generation of the incision, and consequently the entire duration of the operation, may therefore be undesirably long.